A field effect transistor (FET), which is composed of layered nitride semiconductor layers, has been predicted to be a high output power semiconductor device capable of operating at high frequencies with a high breakdown voltage, and a MESFET (Metal Semiconductor FET), a High Electron Mobility Transistor (HEMT) or the like has been proposed. In recent years, a HEMT using a gallium nitride based compound (hereinafter referred to as a “GaN-based HEMT”) has attracted attention as a next generation high speed FET. A GaN-based HEMT has excellent properties compared with a Si-based compound, a GaAs-based compound or the like, as will be described below;
1. a wide band gap, a high saturated electron velocity estimated from an electron effective mass;
2. a high breakdown electric field;
3. stability at a high temperature allows wide application areas, such as capability to be used in a high temperature region such as in the vicinity of an internal-combustion gas engine; and
4. a gallium nitride based compound semiconductor itself, which is a raw material, has advantages such as it is basically a nonpoisonous material, so that it is possible to provide a high frequency device with higher output power and a high breakdown voltage that can operate at high temperature.
An example of a GaN-based compound HEMT shown in FIG. 1A comprises an undoped GaN layer 13 which is a carrier transit layer, an undoped AlGaN layer 18 which is a spacer layer suitably securing high mobility even when the wave function of the channel permeates, and an n-type AlGaN layer 14 which is a carrier supply layer (electron supply layer) sequentially stacked on a sapphire substrate 11 via a GaN buffer layer 12. The spacer layer controls the electronic influence of the impurity ions that have released electrons not to reach the channel. Therefore, electrons can transit through the channel without the influence. In addition, on the upper surface of the n-type AlGaN layer 14, a source electrode (hereinafter referred to as a “S-electrode”, also simplified as “S”) 15, a gate electrode (hereinafter referred to as a “G-electrode”, also simplified as “G”) 16, and a drain electrode (hereinafter referred to as a “D-electrode”, also simplified as “D”) 17 are respectively formed. In this HEMT, the layer 14 supplies electrons to the layer 13 and the supplied electrons form a channel in the region 13a which is in contact with the layer 14 in the uppermost layer of the layer 13. When the drain voltage is applied, the electrons are injected to the carrier supply layer 14 from the S-electrode 15. Then, the electrodes transit in the channel 13a with high mobility, and flow into the D-electrode 17 through the layer 14 again. The channel region is controlled according to the voltage applied to the G electrode. Therefore, by controlling the voltage applied to the G-electrode, the amount of electric current between the S-electrode and the D-electrode can be controlled.    Patent Document 1: Japanese Patent Laid-Open Publication No. 2003-045898A    Patent Document 2: Japanese Patent Laid-Open Publication No. H9-082693A    Patent Document 3: Japanese Patent Laid-Open Publication No. H5-218099A    Patent Document 4: Japanese Patent Laid-Open Publication No. H9-064341A    Patent Document 5: Japanese Patent Laid-Open Publication No. 2003-258005A    Nonpatent Literature 1: M. Miyoshi et al., Jpn J. Appl. Phs., Vol. 44, No. 9A (2005), p6490-6494    Nonpatent Literature 2: D. Oiao et al., Applied Physics Letters, Vol. 80, No. 6 (2005), p. 992-994